With a view towards cost saving and efficiency in manufacturing, in recent years, the use of a vacuum pressure impregnation (VPI) of insulating resin for form wound motor coils has become increasingly prevalent. In a typical VPI process, a previously insulation-wrapped coil, either individually or in a stator, is processed by: (a) preheating it; (b) subjecting it to a vacuum for a predetermined period of time; (c) introducing the insulating resin in liquid form into the coil under vacuum; (d) increasing the pressure on the coil in the presence of the liquid resin until it substantially impregnates the coil; (e) releasing the pressure and draining off any resin which does not so impregnate; and (f) baking the impregnated coil.
The use of such impregnation method is intended to introduce the impregnating insulating resin into all the existing interstices of the coil and insulating coil wrapping, since unimpregnated areas result in voids which cause increased dielectric breakdowns or lower dielectric breakdown voltages.
Since the junction points at which the coil leads are brazed to the form wound coil must be insulated, many manufacturers have been using multiple layers of sleeving to obtain such insulation. Typically, either one of two combinations is used to insulate this junction point and the lead itself: either a heat treated fiberglass sleeving placed over Grade A acrylic resin coated fiberglass sleeving (by American Society for Testing and Materials (ASTM) D 372 standards sleeving which can withstand 7,000 volts average impressed voltage without breakdown), or heat treated fiberglass sleeving placed over two lengths of Grade A acrylic resin coated fiberglass sleeving which previously have been "telescoped" (inserted one into the other).
Although these methods of insulating the junction points are very labor intensive as well as relying heavily on operator skill and conscientiousness, if a VPI process is used, there has been very little alternative to this telescoping method. It has not been possible to use a single Grade A acrylic resin coated sleeving because the insulating impregnating resin does not adhere sufficiently to the acrylic resin coated sleeving to insulate the coil lead adequately. It has therefore been necessary to add a layer of heat treated, uncoated fiberglass sleeving to the lead insulation to which the VPI insulating resin would readily adhere, in order to obtain a final sleeving insulation which provides adequate mechanical as well as electrical protection.
One approach to simplifying and improving the process of insulating such coils has been to form the insulating sleeve into a unitary unit by coating the exterior of an electrically insulating inner layer with an electrically insulating polymer, then overbraiding that insulating polymer with a second electrically insulating sleeving and bonding the second insulating sleeving to the polymer with an impregnated polymer adhesive. The improvement is more fully disclosed in commonly assigned U.S. Pat. No. 4,389,587 and is herein incorPorated by reference.
However, this unitarY sleeving required the extra manufacturing step of impregnating the second insulating sleeve to bond it to the polymer insulation layer, thereby entailing labor costs and potential quality control problems for this item. The use of the impregnating polymer adhesive slightly inhibits the overall performance of the final product in two ways. First, it bonds the two layers together which makes for a stiffer product and second, the resin slightly reduces the amount of VPI resin which is absorbed into the glass. Additionally, the use of these adhesives limits the useful temperature ranges which can be achieved with this type of construction. This is important as advancements in motor generator design, require a higher temperature classification sleeving than the unitary sleevings constructed in the manner (class 155 acrylic sleeving) which is the base for the product described in U.S. Pat. No. 4,389,587.
Therefore, what is needed in this art is a high temperature unitary sleeving which has a super absorbent layer on the outside as well as a product that is very flexible.